Sky & Telescope - September 2020 - 21

VOLCA NIC PIT ON MERCURY: N ASA / JHU A PL / CA R NEGIE
INSTIT U TION OF WASHINGTON; LUN A R VOLCA NISM: N ASA /
GSFC / A RIZON A STATE UNIV ERSIT Y

that the majority of smooth, low-lying plains - distributed
widely across the planet, occupying around a quarter of the
surface, and often situated within impact basins - consist
of lavas that poured across the landscape as quickly moving
ﬂows before solidifying.
Messenger also returned images of irregularly shaped pits
that look like sites of explosive volcanism, where gas-rich
magmas erupted violently instead of as gentle ﬂows. Most
such sites sit in impact craters and basins, or along major
tectonic structures. But notable by their absence are the large
volcanoes that are so widespread on Earth and Venus, and
that form the biggest mountains on Mars.
Messenger told us something else intriguing, too: All of
the planet's major volcanic units were in place by around
3.5 billion years ago. Although explosive activity and smaller
lava ﬂows took place thereafter (some within the last billion
years or so), most of Mercury's volcanic history was complete
within the ﬁrst quarter of solar system history.
Messenger revealed that Mercury bears more than a
passing resemblance to our Moon, particularly in terms of
volcanic characteristics. The Moon also lacks giant volcanic
constructs - it has some short peaks and domes, but the few
low swells that might be shield volcanoes are so broad they're
all but undetectable without high-resolution topography. Like
Mercury, the dominant volcanic features of the Moon are the
vast basaltic lava plains, the lunar maria. Most of the maria
lie in earlier impact basins, just like Mercury's ﬂood plains.
Explosive eruptions also took place on the Moon - but, as
for Mercury, most examples are within and around the edges
of impact features. And, like Mercury, the majority by volume
of the lunar maria had formed by around 3 billion years ago.
Younger ﬂows are found mainly in a region called Oceanus
Procellarum, where heat-producing radioactive elements are
concentrated. Some lavas may be as geologically recent as
within the last 100 million years, but these ﬂows are tiny
compared to their older counterparts.

Total
planetary volume
(10 billion km3)

Crust and
mantle volume
(10 billion km3)

Planetary shrinkage
due to interior cooling
(km)

108.32

90.67

92.85

78.76

16.32

13.67

0.2-3.8

6.08

2.60

4.7-7.1

2.20

2.18

at least 0.07

Shrinking Planets
It turns out, then, that Mercury and the Moon are volcanic
cousins, if not quite siblings. But when we compare their
volcanic characteristics with those of Earth, Venus, and
Mars, we can see just how different they are from their larger
planetary kin.

p PAST EXPLOSION The orange tint and irregularly shaped depressions
in this enhanced-color image of Mercury's 69-km-wide Navoi Crater suggest explosive volcanism once happened here. Sunlight from bottom.

Underlying
material

Smooth material on top

1 kilometer

p LUNAR VOLCANISM Smooth flows overlie older, lighter surface material in this volcanic deposit near the crater Maskelyne (sunlight is coming
from the right). Regions like this one, called irregular mare patches,
appear to be places where small amounts of basaltic lava erupted to the
surface roughly 100 million years ago, long after major volcanic activity
ended on the Moon.
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Sky & Telescope - September 2020

Table of Contents for the Digital Edition of Sky & Telescope - September 2020